1. Field of the Invention
The present invention relates to semiconductor laser devices and particularly to self-pulsing, low-noise semiconductor laser devices.
2. Description of the Background Art
Light of a short wavelength around 400 nm will be used for a light source in an optical disk apparatus of a next generation, because it makes it possible to reduce a focused light beam spot size and thus enables high-density recording. On the other hand, a lens, an optical disk and the like are formed with inexpensive plastic-based material to reduce their costs. Such plastic-based material has an absorption edge wavelength of about 390 nm. Therefore, if the wavelength of the light source is made too shorter than 400 nm, it becomes necessary to carefully consider material for the lens and the like and thus the too short wavelength would not be preferable for mass production of optical disk apparatus.
For a light source of a short wavelength of about 400 nm, semiconductor lasers are used, which are typically formed with gallium nitride compound semiconductor. For example, Japanese Patent Laying-Open No. 10-294532 discloses a semiconductor laser for an optical disk apparatus, which is formed with gallium nitride and has a structure as shown in FIG. 11. In FIG. 11, provided over a sapphire substrate 70 are an n-type GaN buffer layer 71, an n-type GaN contact layer 72, an n-type AIGaN clad layer 73, an adjacent n-type InGaN/GaN multiquantum well layer 74, an active InGaN/GaN multiquantum well layer 75, an adjacent p-type GaN layer 76, a p-type AlGaN clad layer 77, a p-type GaN contact layer 78, and an n-type GaN current barrier layer 79. Furthermore, a p side electrode 80 is provided on p-type GaN contact layer 78 and an n side electrode 81 is provided on n-type GaN contact layer 72 which is partially exposed by anisotropic etching. In this semiconductor laser, island regions 82 with high In concentration in adjacent layer 74 serve as saturable absorption regions for causing self-pulsation.
Japanese Patent Laying-Open No. 9-191160 discloses a semiconductor laser having an InGaN saturable absorption layer, structured as shown in FIG. 12. In FIG. 12, successively provided over an n-type SiC substrate 60 are an n-type AIN layer 61, an n-type AlGaN clad layer 62, an n-type GaN optical guide layer 63, an InGaN quantum well active layer 64, a p-type GaN optical guide layer 65, a p-type AIGaN clad layer 66, and a p-type GaN contact layer 67. Further, an InGaN saturable absorption layer 68 is provided inside p-type GaN optical guide layer 65. Furthermore, an n-type electrode 59 is provided on a back surface of substrate 60 and a p-type electrode 69 is provided on p-type contact layer 67.
Conventionally in an energy band structure of InGaN used for saturable absorption, heavy holes have large effective mass and its valence band has an upper portion of a large state density and thus there hardly occurs saturation of absorption regarding light from an active layer. As such, in a semiconductor laser having a suturable absorption layer of InGaN as described in Japanese Patent Laying-Open No. 9-191160, saturable absorption effect hardly occurs at low output and thus an output of a relatively high level is required to maintain self-pulsation. Therefore, if such a semiconductor laser is used as a light source for an optical disk, it is disadvantageous in its power consumption and its lifetime.
An object of the present invention is to provide a semiconductor laser having a good characteristic of self-pulsation even at low output.
To overcome the disadvantage as described above, the present inventors considered introducing a nitride-based semiconductor layer improved in composition for saturable absorption. As a result, it was found that a gallium-nitride-based semiconductor layer containing at least one element selected from the group consisting of As, P and Sb, can be used to cause saturable absorption and then to provide a semiconductor laser capable of self-pulsing at low output.
Specifically, according to the present invention, a semiconductor laser device has a stacked-layer structure mainly of gallium-nitride-based semiconductor for laser excitation and includes an active layer and a layer greater in bandgap energy than the active layer, wherein the device also includes a gallium-nitride-based semiconductor layer substantially equal in bandgap to the active layer and containing at least one element selected from the group consisting of As, P and Sb for saturable absorption at a location apart from the active layer and inside or in contact with the layer greater in bandgap energy than the active layer.
The gallium-nitride-based semiconductor can be, for example, GaN, AlxGa1xe2x88x92xN (0 less than x less than 1), InxGa1xe2x88x92xN (0 less than x less than 1), InxGayAl1xe2x88x92xxe2x88x92yN (0 less than x less than 1, 0 less than y less than 1), AlxInyGa1xe2x88x92xxe2x88x92yN1xe2x88x92pxe2x88x92qxe2x88x92rAspPqSbr (0xe2x89xa6x, 0xe2x89xa6y, x+y less than 1, 0xe2x89xa6p, 0xe2x89xa6q, 0xe2x89xa6r, p+q+r less than 1). While the stacked-layer structure in the semiconductor laser device of the present invention is formed mainly with such gallium-nitride-based semiconductor as described above, it can also include AlN, InN, InAlN and other similar III-V compound semiconductors, particularly III-N compound semiconductor.
In the present invention, it is preferable that the saturable absorption layer of gallium-nitride-based semiconductor containing at least one of As, P and Sb is formed with a quantum well structure. Furthermore in the present invention, the layer greater in bandgap energy than the active layer can be a clad layer or an optical guide layer.
Typically in the present invention, the saturable absorption layer is formed of gallium-nitride-based semiconductor represented by an expression AlxInyGa1xe2x88x92xxe2x88x92yN1xe2x88x92pxe2x88x92qxe2x88x92rAspPqSbr, wherein 0xe2x89xa6x, 0xe2x89xa6y, x+y less than 1, 0xe2x89xa6p, 0xe2x89xa6q, 0xe2x89xa6r, and 0.001xe2x89xa6p+q+rxe2x89xa60.5. In the expression, preferably, q+r=0 and 0.005xe2x89xa6p, or q+r=0 and 0.006xe2x89xa6q. This is effective as it can provide a sufficient level of crystallinity for the saturable absorption layer.
It is preferable that the laser device of the present invention further includes an AlGaN layer covering the saturable absorption layer. Furthermore in the present invention, it is preferable that the stacked-layer structure of gallium-nitride-based semiconductor is provided on a GaN substrate.
Furthermore in the present invention, the saturable absorption layer can be lower in crystallinity than the layer greater in bandgap energy than the active layer. Generally, the saturable absorption layer is preferably grown at a lower temperature as compared with the layer greater in bandgap energy than the active layer.
The saturable absorption layer has, for example, a thickness of 0.1 nm to 50 nm, preferably 0.5 nm to 20 nm.
GaNAs, GaNP, GaNSb, GaNAsP and any other similar gallium-nitride-based semiconductor containing at least one of As, P and Sb, hereinafter referred to as xe2x80x9cGaN (As, P, Sb)xe2x80x9d, contains heavy holes smaller in effective mass as compared with InGaN. Furthermore, in the energy band structure of GaN (As, P, Sb), the state density in the valence band is small in the vicinity of the band end. Therefore, when light slightly greater in energy than the substantial bandgap is absorbed, heavy hole saturation occurs more readily in GaN (As, P, Sb) than in InGaN. As such, if a semiconductor laser device includes a saturable absorption layer of GaN (As, P, Sb), it can self-pulse even at low output.
Such a laser can be applied for example to an optical disk system of low power consumption. Such an optical disk system essentially requires a semiconductor laser not only having high quantum efficiency but also self-pulsing even at low output and requires a photoelectric conversion device.
Furthermore, a semiconductor laser device capable of maintaining self-pulsation at higher output can be obtained by providing nitride-based semiconductor layers appropriate in structure, arrangement and number. Such a laser device allows images, sounds and other similar information to be steadily written in an optical disk system.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.